Injection molding

The worked trial's 101.20 mm mold dimension and 100.00 mm conditioned feature give 1.18577% shrinkage relative to the mold. If that shrinkage transfers, a 50.00 mm target needs a 50.60 mm mold dimension (50.00 ÷ (1 − 0.0118577)). Validate transfer across direction, wall section, grade and conditioning before cutting production tooling.

This guide covers reciprocating-screw thermoplastic molding. Identify the injection and clamping stages and the cooling and ejection conditions. Thermoset and liquid-silicone routes require cure control. KEYENCE, What is Injection Molding?, undated

Start with the exact material

Specify supplier, grade, reinforcement, color/additives, approved recycled or regrind content, and storage history. A polymer family name does not define a processing window. Obtain the current grade processing sheet and SDS before setting drying, melt temperature, residence-time limits, or purging practice.

BASF's Ultramid brochure, bearing an August 2013 note, explains that excess moisture can impair appearance and mechanical properties through degradation. It directs processors to product-specific drying recommendations, including differences within its polyamide range. Its reprocessing guidance requires clean, dry material without prior thermal degradation. These are polyamide-specific cautions, not universal moisture limits for every resin. BASF Ultramid, preliminary treatment and reprocessing sections

Conversely, LyondellBasell's polyolefin guide describes its resins as non-hygroscopic, while warning about surface condensation when cold pellets enter a warm molding area. “No routine drying” therefore does not mean “wet pellets are acceptable.” Its retrieved footer identifies document 5979/0715; that printed code is retained without treating the website's crawl date as a revision date. LyondellBasell, A Guide to Polyolefin Injection Molding, material handling

Understand the cycle and its overlapping stages

1. Plasticize and meter: the rotating screw conveys and melts pellets and accumulates the next shot. Barrel heaters, screw work, back pressure, and residence history influence the melt; a heater setpoint alone does not establish actual melt temperature.
2. Fill: the screw advances, moving melt through the nozzle and feed system into the cavities. A controlled velocity profile and a defined transfer point govern the transition to packing.
3. Pack and hold: pressure feeds additional material while the gate remains open, compensating for contraction. Switching too late can overpack; continuing a nominal hold after gate freeze cannot keep adding material through that gate.
4. Cool and recover: heat removal continues while the part develops ejection strength. Screw recovery can overlap part cooling; these are not necessarily sequential additions to cycle time.
5. Open, eject, and reset: release the part without excessive deformation, gate damage, or unacceptable ejector marks.

LANXESS's Processing data for the injection molder, edition 2017-10, correction 2021-06, relates switchover and holding pressure to shrinkage, dimensions, and stress. It illustrates determining gate-freeze time from part-weight response or cavity-pressure behavior. Its cooling-time discussion explicitly defines its timing basis, reinforcing the need to state whether a shop's “cooling time” includes holding. Its material scope is Durethan polyamides and Pocan polyesters/blends; its numerical settings are not generic recipes. LANXESS, sections 3.4–3.5

Tooling and flow determine more than appearance

Gate position influences flow paths, packing access, weld-line location, and orientation. Air must escape ahead of the melt; vents that become obstructed can change a previously stable process. Cooling-channel condition and uneven temperatures influence differential contraction. Adequate draft and an appropriate ejection arrangement help prevent removal damage. LyondellBasell discusses these interactions for polyolefins; its particular vent dimensions and temperature ranges should not be transferred to other materials or tools. LyondellBasell, mold venting, cooling, and ejection guidance

A weld line forms where separated flow fronts meet. Celanese's ©2015 Injection Molding Guide for LDPE and EVA Copolymers, document EVA-016-PlasticsMoldingGuide-TG-EN-1115, recommends addressing weld-line placement during design and keeping it away from highly stressed regions. Its EVA discussion also connects ejector design with the risk of pushing through flexible thin sections. These examples demonstrate why gate and ejection decisions belong in design review, not why every resin should use EVA's tool dimensions or additives. Celanese, mold design and troubleshooting

Shrinkage is directional and condition dependent

Mold shrinkage compares mold and molded-part dimensions at specified conditions. It is not thermal expansion alone: crystallization, packing, restraint, thickness, gate position, and subsequent storage can matter. For fiber-reinforced compounds, orientation makes flow and transverse behavior different. BASF's Ultramid brochure describes direction-dependent shrinkage and warns that test-box values provide guidance rather than exact prediction of a real component. Do not enlarge every cavity dimension by one datasheet percentage and assume a warped or constrained part will conform. BASF Ultramid, shrinkage and aftershrinkage

Worked example: reconcile the shrinkage denominator

This original example assumes a representative trial yields 100.00 mm on a defined feature from a 101.20 mm mold dimension, measured at agreed temperature and conditioning time. Using mold dimension as the denominator:

• Shrinkage = (101.20 − 100.00) / 101.20 = 0.0118577, or 1.18577%.
• If that same shrinkage were applicable to a 50.00 mm target, the calculated mold dimension would be 50.00 / (1 − 0.0118577) = 50.60 mm.
• Simply multiplying the target by 1.0118577 gives 50.5929 mm, approximately 0.0071 mm smaller.

This is arithmetic, not authorization to cut production tooling: transfer of shrinkage across dimensions, directions, wall sections, or grades must be established. KEYENCE uses the mold dimension as the denominator in its shrinkage formula. KEYENCE, Molding Shrinkage Calculation

Diagnose defects before changing settings

These are investigation prompts. BASF's engineering-thermoplastic troubleshooter distinguishes these defect families; RTP similarly cautions that no single solution resolves every molding problem. Record changes and verify their effects on other characteristics rather than applying a troubleshooting arrow as a universal instruction. BASF troubleshooter, undated, RTP troubleshooting guidance, undated

Define finished-part acceptance

Agree on dimensions and conditioning time, warpage, gate vestige, flash and cosmetic boundaries, weld-line-sensitive mechanical tests, and application-specific sealing or assembly performance. Inspect identifiable cavities separately so a good average cannot conceal one defective cavity. Resin datasheet properties and an acceptable-looking surface do not replace representative finished-part testing.

A useful production handoff records material lot and preparation, tool/cavity identity, validated process window, actual observations, sampling plan, measurement conditions, and disposition of startup or interrupted-run parts. Treat this as a proposed control framework to adapt to the product's requirements, not a certification implied by the cited processing guides.

Related reference

Manufacturing processes